The contamination of the environment by pollutants is a growing problem that is receiving increased recognition. Industrial and manufacturing processes are a major source of contaminating material in the environment and generate numerous materials that present disposal problems. In some cases, such materials may be ignitable, corrosive, unstable, radioactive or toxic, rendering them hazardous. In other cases, the materials may be non-hazardous (e.g., acetic acid or cooking oil) but may be present at levels which present disposal problems. In either case, disposal of such materials is often subject to extensive government regulation, which can make their treatment, storage and disposal very costly.
Toxic organic compounds, heavy metals and radioisotopes often pose particularly difficult problems for disposal. Compositions containing even low levels of such components may be classified by government regulatory agencies as hazardous materials. For example, in the United States, the Environmental Protection Agency (EPA) has set limits on the disposal of specific toxic compounds (see, e.g., 40 C.F.R. .sctn.61.24, and Table 1 therein). Any material that contains more than the specified limit of any listed toxic compound is defined as hazardous, and is subject to federal regulations controlling its handling and disposal. The EPA's lists of toxic compounds are quite extensive and much of the material generated by industrial processes is subject to government regulation.
In recent years, numerous methods for treating hazardous materials so as to render them less-hazardous have been developed. Perhaps the most common treatment is incineration. In this process, a material is heated in the presence of oxygen at sufficiently high temperatures such that the material undergoes combustion and breaks down into smaller, oxygen-containing components. Oxygen, sometimes in the form of air, is present in all incinerators in at least stoichiometric amounts (and generally in excess) so that complete combustion of the material can occur.
Some incinerators consist of a single incinerating chamber. In such incinerators, the material may be sprayed into an open flame, as in U.S. Pat. No. 3,734,035, or onto a heated hearth within the incineration chamber, as in U.S. Pat. No. 3,748,081. Other incinerators contain multiple sequential chambers that operate at successively higher temperatures. This design provides for more complete incineration of the material using less support fuel, since the gaseous materials generated at a lower temperature may contain combustible reactants which supplement and/or promote burning in the later chambers. An example of a multiple-chamber system for hazardous waste incineration is disclosed in U.S. Pat. No. 4,398,475. That system consists of a series of combustion chambers in which the material is burned at successively higher temperatures, and oxygen is supplied to each chamber in a stoichiometric amount to ensure complete combustion. Modifications may also be made to the incineration process specific for particular materials. For example, a catalyst may be added, as in U.S. Pat. No. 3,881,430, or a reducing agent may be included, as in U.S. Pat. No. 5,269,235.
Less frequently, a pyrolysis step is employed prior to combustion of the hazardous material. Pyrolysis is the heating of a material in the absence of oxygen. The products of pyrolysis depend upon the temperatures employed, but organic compounds generally form free radicals, which coalesce to form particulate materials (e.g., solid residues such as carbon black or char). In the process, hydrogen gas (H.sub.2) is released. The use of a pyrolysis step in the treatment of hazardous materials is discussed, for example, in U.S. Pat. Nos. 4,279,208, 4,182,246 and 4,179,263. The treatment processes of those patents involve pyrolysis of organic compounds in an atmosphere with a low oxygen content to achieve partial oxidation of the compounds prior to incineration. Such a step apparently results in more efficient and complete combustion of the organic compounds during subsequent incineration. Pyrolysis has also been used to treat materials without incineration, since the particulate materials that remain after pyrolysis may be easier to dispose of than the original material.
Pyrolysis is most commonly used to break down large organic compounds to smaller hydrocarbon products. For example, in a form of pyrolysis known as cracking, large hydrocarbons are broken down to smaller hydrocarbons by heating in the absence of water or oxygen. In this process, random carbon-carbon bonds of the large hydrocarbons cleave to produce smaller free radicals which, in turn, generate the smaller hydrocarbons. For example, when a large alkane is pyrolized in this manner, the product is a mixture of smaller alkanes and alkenes. In a related process, known as steam cracking, large hydrocarbons can be cleaved to form smaller hydrocarbons by heating in the presence of relatively small amounts of water (generally 20% to 50% by weight). The products of steam cracking are most commonly olefins and aromatic compounds. Like cracking in the absence of water, steam cracking is used to generate hydrocarbon products.
Current methods for treating hazardous materials by incineration suffer from numerous inherent problems, including the emission of potentially toxic materials, inefficiency, inconvenience and high cost. The use of a pyrolysis step prior to incineration has not overcome these difficulties, and the use of pyrolysis as the sole method of treatment is limited to materials having a low water content. Furthermore, the treatment of chelated metals and radionuclides, by both incineration and pyrolysis methods, is particularly problematic because of the difficulty associated with separating the metal or radionuclide from the end product of such treatment. The separation from the chelating agent is important for minimizing the volume of the hazardous material, or for preparing a metal for reuse, but such separation has been difficult to achieve.
An alternate method for treating some types of organic materials is a process known as thermolytic detoxification. In this process, described in U.S. Pat. No. 4,874,587, as well as PCT Publication No. WO 90/10367, organic compounds are first vaporized by heating. The vaporized organic compounds are then combined with steam and heated to a high temperature. The mixture of steam and organic compounds is also passed through a heated, labyrinthine path which contains organically adsorbent materials which adsorb the organic compounds prior to subjecting the same to the high temperature. Water is present in excess of stoichiometry, but at a level that is less than 200% of stoichiometry and preferably 131% of stoichiometry. The combination of steam and organic compounds is recirculated in the system until treatment is complete. In a related process for treating medical waste, described in PCT Publication No. WO 94/20149, the waste is vaporized in superheated steam prior to treatment according to the above process.
In spite of the apparent advantages of thermolytic detoxification over existing incineration and pyrolysis techniques, the utility of this process is limited to a narrow range of organic compounds. For example, highly flammable or explosive materials cannot be treated by this technique, since the initial vaporization and/or treatment at elevated temperatures may cause fire or explosion. For organic compounds that may be decomposed by this method, such decomposition remains costly, since relatively high temperatures are required for treatment. In addition, inorganic materials (such as metals and inorganic salts) and radioactive materials are not suitable for treatment by this procedure.
Accordingly, there is a need in the art for a method and apparatus for treating materials that present disposal problems, and which are not subject to the limitations associated with existing incineration, pyrolysis and detoxification techniques. The present invention fulfills these needs, and provides further related advantages.